Title: Studying the Dark Universe with Gravitational Lenses

Speaker: Kenneth Wong (National Astronomical Observatory of Japan)

Time: Jan 29 (Monday), 3pm

Location: room 1715, 17th floor

Abstract: Strong gravitational lensing is a powerful probe of the mass distribution in the Universe. Lensing is sensitive to the total mass distribution along the line of sight, making it a unique probe of dark matter in lensing galaxies. However, strong lenses are quite rare and require deep wide-area imaging surveys to build up a statistical sample. The Hyper-Suprime Cam survey is an ongoing multiband imaging survey using the Subaru Telescope that will cover 1400 deg^2 of the sky to a depth of r~26. I present the current work of the HSC SSP strong lens working group, which is focused on searching for new lenses and leveraging these systems for studies of galaxy structure and cosmology. The search methods and science cases being developed now for surveys such as the HSC SSP are necessary to prepare for the upcoming revolution in strong lensing from LSST and Euclid, which will discover orders of magnitude more lenses than are currently known. Searches for lensed quasars are particularly valuable because they are variable and can be monitored to measure the "time delay" between the multiple images. In particular, the time-delay distance from such a system is primarily sensitive to the Hubble constant (H0). This method is independent of type Ia supernovae and CMB observations, and may shed light on the growing H0 discrepancy between local universe and CMB measurements. I discuss the H0 Lenses In COSMOGRAIL’s Wellspring (H0LiCOW) project, which has measured H0 to ~3.8% precision for a flat Lambda CDM cosmology from three time-delay lenses. Our results are in moderate tension with the latest Planck results for a similar cosmology, hinting at possible new physics beyond the standard LCDM model and highlighting the importance of this independent probe.